This invention relates to an information read apparatus for reading information tracks in an information plane of an information carrier, with the aid of a tracking control system, and more particularly with the aid of a tracking control system comprising a first closed loop for high speed control, and a second closed loop for high precision control.
Tracking control systems which usually consist of single closed loops need a means for detecting tracking error. A differential method and a wobbling method are both well known means for detecting tracking error.
Said differential method is typical. In this method, a tracking error is detected differentially by plural detectors. This is discussed in detail in U.S. Pat. No. 3,909,608. This method is very simple and easy, but has a defect in that the detected tracking error signal inherently tends to include low frequency (or DC) noises, due to the fact that said detectors detect a variation of some physical value caused by a tracking error instead of detecting the tracking error directly. Generally speaking, said detected value does not always precisely coincide with the tracking error. For example, in an optical information readout system, a reflected light beam from an information carrier forms a far-field-pattern around the boundary of plural photodetectors neighboring each other, and the boundary is effectively parallel to an information track. When a readout light spot deviates from the center of an information track, the power distribution of the far-field pattern becomes unbalanced about the boundary. Then, a tracking error can be detected as the differential signal from the plural detectors. The power distribution of the far-field pattern, however, becomes unbalanced not only when there is a tracking error but also when the information carrier is warped. Therefore, in this method, a false tracking error tends to be detected. Moreover, in many cases, the tracking error signal is so small that the tracking control is disturbed by a DC-offset of the head amplifier which is operatively coupled to said detectors.
On the other hand, said wobbling method does not have the defect mentioned above. In this wobbling method, a tracking error is detected as follows. The reading position on the information track being read moves periodically at a properly selected frequency about a global position with a low amplitude (wobble) in a direction traverse to said information track. When there is no tracking error about the global reading position, the amplitude of the read out signal varies periodically at double the wobbling frequency. But when a tracking error occurs, the variation of the amplitude of the read out signal includes the same frequency component as the wobbling frequency. The phase of said same frequency component is reversible relative to the phase of wobbling according to the direction of the tracking error. Accordingly, a tracking error can be detected from the amplitude signal by synchronous detection with reference to the wobbling signal, where said amplitude signal is the signal which is proportional to the amplitude of said read out signal from the read head. Further details of the wobbling method are described in U.S. Pat. No. 4,151,570.
In the above wobbling method, a tracking error is directly detected through amplitude reduction of the read out signal. Therefore, the detected tracking error signal precisely coincides with the real tracking error. Besides, tracking error signals obtained through synchronous detection are not disturbed much by DC-offsets of the pre-amplifier, because said amplitude signals which are to be synchronously detected have been sufficiently amplified after passing through the bandpass filter whose center frequency is equal to the wobbling frequency.
The wobbling method, however, has a defect in that the reading position must be wobbled at a frequency which is substantially higher than the cross-over frequency of the tracking control system. If the cross-over frequency is required to be as high as 1 KHz, the wobbling frequency is required to be higher than about 10 KHz. Therefore, in this case, a special actuator is usually necessary for wobbling. When the reading position is wobbled at a relatively low frequency by the same actuator that controls the global reading position, the tracking control system can not follow high speed deviation because the cross-over frequency of the tracking control system can not be set so high.